Several processes are known for producing a microcapsule comprising a polymer wall which confines an active material that is liquid at room temperature or a liquid solution of such active material. According to a typical method, a preformed polymer is used as a wall-forming material and a polymer emulsion is dried in water. If the active material is to be encapsulated as an aqueous solution, the following method is generally used: a film-forming polymer is dissolved in an organic solvent having a lower boiling point than that of water, and the aqueous solution of the active material is added to the polymer solution to make a primary water-in-oil (W/O) emulsion. The emulsion is added under agitation to an aqueous solution containing a protective colloidal substance such as gelatin or surfactant, thereby forming a secondary (W/O)/W emulsion. This emulsion is heated or placed under vacuum to remove the organic solvent and provide a microcapsule having the active material confined by the polymer wall. Such procedures are described in U.S. Pat. No. 3,523,906.
If the active material is an oil which is either insoluble or poorly soluble in water or if a solution of such oil is used as the active material, the following method may be used: the active material and a hydrophobic film-forming polymer that is not miscible with this active material are dissolved in an organic solvent capable of dissolving the two components but which is poorly soluble in water and has a lower boiling point than that of water; the resulting solution is dispersed in an aqueous solution containing a protective colloidal substance such as gelatin or surfactant, thereby providing an O/W emulsion; the emulsion is then heated or placed under vacuum to remove the organic solvent and produce a microcapsule having the active material confined by the polymer wall. Such procedures are described in U.S. Pat. Nos. 3,660,304 and 3,960,757.
These prior art methods of encapsulation have a relatively small latitude for the selection of the film-forming polymer, because it depends entirely upon whether the active material to be encapsulated is oily or aqueous. Furthermore, these methods involve the step of forming the primary or secondary emulsion, and the quality of the secondary emulsion produced is not always satisfactory. As a further disadvantage, the solvent used must be water-insoluble and a long period of time is generally required for removing the solvent.
Pest control with minimum disturbance to the eco-system is becoming an increasingly important goal, and one method effective for this purpose is by slowly releasing sex pheromones into air. Sex pheromones are secreted by glands in the terminal segments of the abdomen of female insects, and can be used to attract male insects into traps where they are killed, or interfere with their normal mating. Microcapsules prepared by the prior art methods are of course effective to some extent for the purpose of slowly releasing pheromones. However, most pheromones are chemically labile because they contain unsaturated bonds or aldehyde groups. Encapsulated pheromones, when exposed to ambient conditions, easily change to other substances which do not have the desired effect. Therefore, it has generally been difficult to prepare microcapsules from which pheromones can be slowly released while exhibiting their ability to attract males and interfere with mating over a long period of time. In particular, with the conventional method of encapsulation, microcapsules are formed in water, and the water or oxygen present in the microcapsules may accelerate the deterioration of the liquid active material, and in an extreme case, that active material may be lost into the water. Entrance of water into the microcapsule may also occur easily when the active material is an oil or a solution thereof.